Chapter 7: Study Guide Summary

Question 1

What are the two main components of magnetic particle inspection materials?

Answer 1

• Particles
• Suspension liquid

Both components must be controlled and meet requirements specified in Aerospace Materials Specifications.

Question 2

Fill in the blank: Magnetic particles can be used in both oil and water if a _______ is added to the water.

Answer 2

wetting agent

Usually a type of detergent is used.

Question 3

What factors can affect the curing time of magnetic rubber?

Answer 3

• Ambient temperature
• Humidity

These factors make it difficult to determine the exact curing time needed.

Question 4

How does density affect the mobility of magnetic particles?

Answer 4

• Metallic and oxide types are about seven times denser than water
• High viscosity can inhibit mobility

The combination of size, shape, and density must be considered with the viscosity of the carrier.

Question 5

What are the basic characteristics of magnetic particles?

Answer 5

• Size
• Shape
• Density
• Contrast and visibility
• Permeability and retentivity

These characteristics influence the effectiveness of magnetic particles in detecting leakage fields.

Question 6

What are the requirements for dry magnetic particles?

Answer 6

• Necessary magnetic properties
• Light and mobile

Elongated particles alone will not be free-flowing; globular particles must also be used.

Question 7

What is the composition of magnetic rubber used in magnetic particle testing?

Answer 7

A two-part synthetic rubber system

Fine black particles are premixed in the rubber component.

Question 8

True or false: Odorous oils are generally acceptable in magnetic particle testing.

Answer 8

FALSE

Odorous oils can indicate undesirable contaminants and are objectionable to operators.

Question 9

What is the purpose of multicolored particles in magnetic particle inspection?

Answer 9

Used under visible light

Fluorescent-colored particles are used under ultraviolet light.

Question 10

What are concentrates in magnetic particle inspection?

Answer 10

Magnetic particles coated with wetting agents

They combine readily with the carrier and can be poured directly into the machine.

Question 11

What are wet particles suspended in?

Answer 11

• Light oil
• Water
• Other suitable and approved carrier

They can be made of even finer powders than dry particles.

Question 12

What is the advantage of the magnetic particle testing process?

Answer 12

Techniques can be tailored to meet specific conditions

Mixing and matching different types of magnetic particles and magnetizing techniques changes the inspection capabilities.

Question 13

What is the minimum closed cup flashpoint desirable for oils used in magnetic particle testing?

Answer 13

57 °C (135 °F)

This threshold provides the lowest practical flammability for oils of the desired viscosity.

Question 14

What type of particles are used in magnetic particle inspection?

Answer 14

Very fine ferromagnetic particles

They have low retentivity, low coercive force, and high permeability.

Question 15

The concentration of a visible powder is about _______ times greater than that of a fluorescent powder.

Answer 15

10

This higher concentration allows for more particles to be attracted to a leakage field.

Question 16

What is the best means for providing mobility to dry powders?

Answer 16

Using pulsating magnetic fields such as half-wave direct current

This enhances the movement of particles for effective inspection.

Question 17

What is the significance of the shape of magnetic particles?

Answer 17

• Elongated particles can result in magnetic polarization
• Globular particles do not clump together
• A mixture provides adequate mobility

The shape affects how particles bridge small leakage fields.

Question 18

What properties of magnetic particles affect their ability to form indications? List them.

Answer 18

• Size (dimension)
• Shape (elongated or globular)
• Density (weight per unit volume)
• Magnetic characteristics (permeability and retentivity)

These properties are critical for effective inspection.

Question 19

What are the effects of using light oil carriers with magnetic particles?

Answer 19

• Compatible with 60 cycle alternating current
• Effective with 0.5 s and 1.0 s interval of stationary magnetizers

Light oil carriers help maintain particle mobility.

Question 20

How is magnetic rubber used to inspect parts?

Answer 20

How It’s Used in NDT:

The surface of the test object is cleaned.
A magnetic field is applied to the area of interest using permanent magnets or electromagnets.

Application:
The magnetic rubber compound is poured or injected into cavities, holes, or complex geometries (like bolt holes, threads, or grooves). It is allowed to cure and solidify while under the magnetic field.

Inspection:
After curing, the rubber is removed and examined under appropriate lighting. Any discontinuities (like cracks or voids) will cause the magnetic particles to accumulate, forming visible indications on the rubber surface.

Question 21

What are the characteristics of water suspensions used in magnetic particle testing?

Answer 21

• Treated with detergent
• Contains antifoam agent
• Incorporates rust inhibitors
• Must be used above freezing

Water suspensions are popular due to flammability concerns with oil but require careful management.

Question 22

What happens if dry magnetic particles are too fine?

Answer 22

They cause false indications or too much background, masking indications

If too large, leakage fields will not hold the particles.

Question 23

What do fluorescent dyes provide when used with magnetic particles?

Answer 23

• Maximum contrast under ultraviolet light
• Emits visible light with minimal background interference

Fluorescent dyes enhance visibility in darkened areas.

Question 24

True or false: Magnetic rubber can be reused multiple times.

Answer 24

FALSE

Magnetic rubber is used only one time, and its constituent ages in the can.

Question 25

What does a **hysteresis curve** indicate about magnetic particles?

Answer 25

* High retentivity: easily magnetized * Low retentivity: easily removed from test object ## Footnote The shape of the hysteresis curve reflects the particle's ability to hold magnetization.

Question 26

What is a **water break test** used for in magnetic particle testing?

Answer 26

To ensure sufficient wetting agent in the suspension ## Footnote This test should be performed at least daily.

Question 27

For new oil, the **kinematic viscosity** at 38 °C (100 °F) should not exceed _______.

Answer 27

3 mm2/s (3 centistokes) ## Footnote This viscosity ensures good particle mobility during testing.

Question 28

True or false: **Magnetic particles** are less important than the proper magnetizing equipment.

Answer 28

FALSE ## Footnote Magnetic particles are just as important as the magnetizing equipment and techniques.

Question 29

Magnetic particles used in wet systems range in size from _______ to _______.

Answer 29

60 μm to 40 μm ## Footnote This size range is critical for their attraction to different leakage field strengths.

Question 30

What are the **characteristics** that must be considered when using **oil** for suspending magnetic particles?

Answer 30

* Viscosity * Flammability * Odor * Color * Fluorescence * Reactive constituents * Contamination ## Footnote Each characteristic affects the performance and safety of the magnetic particle testing process.

Question 32

What was the challenge with **pastes** in magnetic particle inspection?

Answer 32

Difficult to break down and work into a heavy slurry without lumps ## Footnote Many suppliers no longer produce pastes due to the development of dry powder concentrates.